The recovery of alkali metals from geothermal brines represents a resource that has yet to be exploited. At present, most of the commercially obtained sodium (Na) and potassium (K) comes from large-scale underground or solution mining of massive salt deposits found in evaporated basins. Ores are beneficiated by flotation, dissolution-recrystallization (with solar evaporation), heavy-media separations, or combinations of these processes.
On the other hand, trace amounts of lithium (Li) are commercially recovered from non-geothermal sources. The recovery process involves evaporative concentration and the unique (among the alkali elements) ability to precipitate the lithium as a relatively insoluble carbonate salt.
By contrast, both cesium (Cs) and rubidium (Rb) are typically produced commercially by “hard rock” mining of a relatively rare, highly differentiated derivative of granite, known as a pegmatite. Deposits are typically small and ore dressing generally involves dissolution in highly aggressive acid mixes followed by various chemical processes to separate Cs and Rb from the more abundant K, Na and Li that also are found in such rocks.
None of the approaches described above provides a pre-existing methodology which could be directly adapted to recovering Cs and Rb from geothermal brines. Instead, adding geothermal brines to the potential resource base for obtaining these elements requires looking beyond the current state-of-the-art in the mining industry. Environmental concerns, particularly those related to the nuclear industry, have provided a venue where significant advances in Cs (and, hence, Rb)-recovery technologies have been made.
Industry concerns range from issues of pre-concentrating ultra-trace 137Cs levels associated with fallout to issues of treating the rather significant amounts of radio-cesium associated with nuclear wastes. In aggregate, a wide variety of possibilities can be derived from the literature related to the recovery and treatment of radio-cesium. For example, numerous effective organic-based complexing agents and resins have been described with this application in mind. For each material, however, important issues remain to be addressed, such as thermal stability, ultimate uptake capacity, reversibility of sorption, selectivity for target metals over more abundant constituents dissolved in geothermal fluids, and cost.
Thus, further advancements are needed in the area of alkali metals recovery.